Enhancement of Performance and Mechanism Studies of All-Solution

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

The Enhancement of Performance and Mechanism Studies of All-solution Processed Small-Molecule based Solar Cells with an Inverted Structure Guankui Long, Bo Wu, Xuan Yang, Bin Kan, Ye-cheng Zhou, Li-Chuan Chen, Xiangjian Wan, Hao-Li Zhang, Tze Chien Sum, and Yongsheng Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b05317 • Publication Date (Web): 09 Sep 2015 Downloaded from http://pubs.acs.org on September 10, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

The Enhancement of Performance and Mechanism Studies of All-solution Processed Small-Molecule based Solar Cells with an Inverted Structure

Guankui Long1†, Bo Wu2†, Xuan Yang1, Bin Kan1, Ye-cheng Zhou3, Li-chuan Chen3, Xiangjian Wan1, Hao-li Zhang3, Tze Chien Sum2*, and Yongsheng Chen1*

1

State Key Laboratory and Institute of Elemento-Organic Chemistry and Centre for

Nanoscale Science and Technology, Institute of Polymer Chemistry and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin, 300071, China 2

Division of Physics and Applied Physics, School of Physical and Mathematical

Sciences, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore 3

State Key Laboratory of Applied Organic Chemistry (SKLAOC), College of

Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, China

KEYWORDS: organic photovoltaic, small molecules, inverted device, transient absorption spectra, electrical simulation

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Both solution-processed polymer and small molecule based solar cells have achieved PCEs over 9% with the conventional device structure. However, for the practical applications of photovoltaic technology, further enhancement of both device performance and stability are urgently required, particularly for the inverted structure devices, since this architecture will probably be most promising for the possible coming commercialization. In this work, we have fabricated both conventional and inverted structure devices using the same small molecular donor/acceptor materials and compared the performance of both device structures, and found that the inverted structure based device gave significantly improved performance, the highest PCE so far for inverted structure based device using small molecules as the donor. Furthermore, the inverted device shows a remarkable stability with almost no obvious degradation after three months. Systematic device physics and charge generation dynamics

studies,

including

optical

simulation,

light-intensity-dependent

current-voltage experiments, photocurrent density-effective voltage analyses, transient absorption measurements, and electrical simulations, indicate that the significantly enhanced performance using inverted device is ascribed to the increasing of Jsc compared to the conventional device, which in turn is mainly attributed to the increased absorption of photons in the active layers, rather than the reduced non-geminate recombination.

1. INTRODUCTION Solution processed solar cells have received great research interests as a clean and competitive renewable energy source due to their attractive features such as low-cost, light weight, solution processability, high mechanical flexibility, rapid energy payback time and etc.1-4 According to the charge transport directions, organic photovoltaics (OPVs) could be divided into conventional and inverted device structures,5, 6 where the inverted structure based devices are most likely the case for the final commercial applications.7 Since the report for the first inverted device in


Page 2 of 30

Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2005,8 great progress has been made,9-14 and recently, the PCE for polymer-based OPV (P-OPV) have improved to over 9% by several groups with inverted structure devices.15-19 Meanwhile, small molecule based OPVs (SM-OPVs) have also stimulated great attention,20-24 owing to its well-defined molecular structures, intrinsic monodispersity, high purity, less batch to batch variation, more easily controlled energy levels, absorption and etc.,25-45 compared with the P-OPVs. Very recently, the PCE for conventional SM-OPV devices has reached over 9%.27, 35,

46

However, for the more

commercial-related inverted devices with generally higher stability, the much deserved attention for SM-OPVs is not forthcoming. Hence, little is known about the device structure and performance relationships for inverted structure based devices using small molecules. Considering the emergence of SM-OPVs and their advantages, it is of paramount importance to systematically investigate the impact of inverted device structures on SM-OPVs and further optimize the device performances. Our group has reported a series of acceptor-donor-acceptor (A-D-A) type oligothiophene derivatives which exhibited very promising performance for solution-processed OPV devices.35,

36, 46-49

Among these molecules, compound

DRCN7T (2,2'-((5Z,5'Z)-5,5'-((3,3'''',3''''',3'''''',4',4''-hexaoctyl-[2,2':5',2'':5'',2''':5''',2'''':5'''',2''''':5''''' ,2''''''-sepithiophene]-5,5''''''-diyl)bis(methanylylidene))bis(3-ethyl-4-oxothiazolidine-5, 2-diylidene))dimalononitrile, the structure is shown in Figure 1) gave one of the best PCEs so far for small molecule based OPV using PFN as the electron-transporting layer (ETL),46 however, the stability of PFN based conventional device is proved to be very poor.15 So in this work, using this material as the donor material, we have fabricated more stabilized devices employing ZnO nanoparticles (NPs) as the ETL in both conventional and inverted structure devices under similar conditions, and systematically investigated and compared their performances. Based on these systematic investigations, it is found that the inverted device structure significantly improves the Jsc, with a PCE of 8.84% achieved compared to 8.06% for conventional device under similar fabrication conditions using ZnO NPs as the ETL.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Note that the PCE of 8.84% is the highest efficiency reported for small molecule-based solar cells with inverted structures.28, 50-52 Based on the mechanism studies

through

techniques

of

optical

simulation,

light-intensity-dependent

current-voltage experiments, photocurrent density-effective voltage analyses, transient absorption measurements, electrical simulations and etc., we found that the enhanced performance for the inverted device is mainly attributed to the increased effective photon absorption of the active layer, rather than the reduced non-geminate recombination.

Figure 1. The chemical structure of DRCN7T, and schematic of conventional device with structure ITO/PEDOT:PSS/DRCN7T:PC71BM/ZnO NPs/Al (a) and the inverted device with structure ITO/ZnO NPs/DRCN7T:PC71BM/modified PEDOT:PSS/Ag (b).

2. EXPERIMENTAL SECTION 2.1. Materials preparation All reactions and manipulations were carried out under an argon atmosphere using standard Schlenk techniques. PC71BM was purchased from American Dye Source, Inc. DRCN7T was synthesized using our previously reported method.47, 48 ZnO NPs were prepared using the techniques described in the literature.53 The modified PEDOT:PSS was prepared by diluting PEDOT:PSS (Baytron P VP AI 4083) with equal volume of isopropyl alcohol and 0.2% (weight) Zonyl FSN, and the mixture was stirred overnight before using.


Page 4 of 30

Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2. Device fabrication and characterization The

conventional

devices

were

fabricated

with

the

structure:

glass/ITO/PEDOT:PSS/DRCN7T:PC71BM/ZnO NPs/Al. The ITO-coated glass substrates were cleaned by ultrasonic treatment in detergent, deionized water, acetone, and isopropyl alcohol for 15 minutes each and subsequently dried by blowing nitrogen. A thin layer of PEDOT:PSS (Baytron P VP AI 4083, filtered at 0.45 µm) was spin-coated (3000 rpm, ca. 40 nm thick) onto an ITO surface. After being baked at 150 °C for 20 minutes, the substrates were transferred into an argon-filled glove box. Subsequently, the active layer was spin-casted from blend ratios (w/w, 1:0.5) of DRCN7T (10 mg mL-1) and PC71BM in a chloroform solution at 1700 rpm for 20 sec on the ITO/PEDOT:PSS substrate. ZnO NPs was spin coated onto the active layer and thermal annealing was performed at 100 ºC for 10 minutes. Finally, a 60 nm Al layer was deposited on the active layer under high vacuum (< 1.5×10-4 Pa). The inverted devices were fabricated with the structure glass/ITO/ZnO NPs/DRCN7T:PC71BM/modified PEDOT:PSS/ Ag. A thin layer of ZnO NPs (in n-BuOH, filtered at 0.22 µm) was spin-coated (3000 rpm, ca. 30 nm thick) onto the pre-cleaned ITO surface. After being baked at 120 °C for 15 minutes, the substrates were transferred into an argon-filled glove box. Subsequently, the active layer was spin-casted onto the ITO/ZnO NPs substrate, which was further thermal annealing at 100 ºC for 10 minutes. Then the modified PEDOT:PSS was spin coated onto the active layer and another thermal annealing was performed (100 ºC for 10 minutes). Finally, a 60 nm Ag layer was deposited on the active layer under high vacuum (< 1.5×10-4 Pa). The effective area of each cell was about 4 mm2 defined by masks. The thicknesses of the active layer and ZnO NPs were measured using a Dektak 150 profilometer. Atomic force microscope (AFM) investigation was performed using Bruker MultiMode 8 AFM in “tapping” mode. The transmission electron microscopy (TEM) investigation was performed on a JEOL JEM2010FIF operated at 200 kV. The current density-voltage (J-V) curves of the photovoltaic devices were obtained by a Keithley 2400 source-measure unit. The photocurrent was measured under



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

illumination

simulated

100

cm-2

mW

(AM

1.5G)

Page 6 of 30

irradiation

using

a

xenon-lamp-based solar simulator [Oriel 96000 (AM 1.5G)] in an argon-filled glove box. The irradiance was calibrated using a certified silicon diode. External quantum efficiency (EQE) values of the encapsulated devices were measured using a lock-in amplifier (SR810, Stanford Research Systems). The devices were illuminated by monochromatic light from a 150 W xenon lamp passing through an optical chopper and a monochromator. Photon flux was determined by a calibrated standard silicon photodiode. 2.3. Optical simulations The optical simulation was modeled using a transfer matrix model (TMM), the MATLAB

program

is

available

http://mcgeheegroup.stanford.edu/transfermatrix.

The

online

at

one-dimensional

spatial

distribution of normalized incident light intensity (|E|2) inside the devices was calculated by means of an optical TMM approach. The spatial distribution of the absorbed

photon

flux

density

could

then

be

calculated

by

integrating

single-wavelength |E|2 with an AM 1.5G spectrum from 300 nm to 800 nm. Finally, the theoretical maximum Jsc for a device under AM 1.5G illumination was determined by spatially integrating the absorbed photon flux density within the active layer, assuming 100% internal quantum efficiency for all wavelengths. 2.4. Light intensity experiment In order to investigate the dependence of Voc and Jsc on the light intensity, the intensity of the light was modulated with a series of two neutral density filters wheels of six filters apiece, allowing for up to 17 steps to adjust the intensity from 100 to 5 mW cm-2. 2.5. Transient Absorption Spectroscopy Transient absorption measurements were performed using a commercial Helios and EOS setups (Ultrafast Systems LLC). They were used to measure transient absorption dynamics in the fs-ns and ns-µs time regimes, respectively. The pump pulse (500 nm) was generated from an optical parametric amplifier (OPerA Solo) that was pumped by a 1-kHz regenerative amplifier (Coherent Libra). The beam from the regenerative


Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amplifier has a center wavelength at 800 nm, a pulse width of around 50 fs and a power of 4 mJ per pulse and was seeded by a mode-lock Ti-sapphire oscillator (Coherent Vitesse, 80MHz). The pump beam energy was set to be around 15 µJ/cm2 per pulse. The probe beam for Helios was a white light continuum generated from either a 2 mm sapphire plate for the visible part (400 nm-800 nm) or a 1 cm sapphire plate for the NIR part (800 nm-1600 nm) using 800 nm fs pulse from the regenerative amplifier above. The probe beam for EOS was a white continuum generated from a photonic fiber using a Nd:YAG laser (center wavelength: 1064 nm). The probe beam was collected using a dual detector for UV-Vis (CMOS sensor) and NIR (InGaAs diode array sensor). The delay between pump and probe beam for Helios was altered using a mechanical delay stage that can have a maximum delay time of around 5.8 ns. For EOS, the delay time between pump and probe is electronically tuned and has a maximum time window of 400 µs. 2.6. Quasi-steady-state PIA spectroscopy Quasi-steady-state PIA measurements were performed with a home-built setup using a 532 nm LED source as pump beam and a Xe-lamp or the white-continuum in EOS setup as probe beam. The pump beam was chopped at a frequency of 500 Hz. The probe beam was collected using a dual detector for UV-Vis (CMOS sensor) and NIR (InGaAs diode array sensor).

3. RESULTS AND DISCUSSION The details for the fabrication of both the conventional and inverted devices are described in the Experimental Section. Note that both device types are fabricated under similar processing conditions, including the same active layer, thickness, ETL, hole transporting layer (HTL) and etc. to make sure the comparison reliable. 3.1. Optical Simulation In order to understand the impact of device structures on the light intensity distribution in the active layer, optical simulations were first performed based on real



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

device conditions by using a one-dimensional transfer matrix model (TMM) for both cases.54-56 As shown in Figure S-1, the optical simulation shows that the electric field intensity is distributed differently within the active layer for the conventional and inverted devices. Importantly, in `the case of inverted structure, this optical simulation predicts an improved absorption, such as 6.3% improved absorption at the maximum absorption (621 nm) position of the donor material, which is expected to increase the short circuit current density (Jsc) accordingly. This can be observed more clearly in the simulated exciton generation rates57 for both devices shown in Figure 2, which gives the extremal Jsc,simu (assuming internal quantum efficiency (IQE)=100%) of 15.03 and 16.13 mA cm-2 for the conventional and inverted structure device (as shown in Figure S-2), respectively. Therefore an improved PCE would be expected for the inverted structure device if other parameters such as open circuit voltage (Voc) and fill factor (FF) could be similar in both cases.

Figure 2. The simulated exciton generation rates in the studied inverted device (a) and conventional device (b) as a function of depth of the active layer calculated using a one-dimensional transfer matrix model. The incidence light was entered the device from the left of the figure (0 nm at the x-axis).

3.2. The Electron and Hole Transporting Layers


Page 8 of 30

Page 9 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Based on the optical simulation results, we fabricated both conventional and inverted device with DRCN7T:PC71BM as the active layer (as shown in Figure 1). Considering the complicated preparation process and high post-thermal annealing temperature (varies from 200 to 450 °C in literatures) for sol-gel ZnO,9, 10, 14, 28 ZnO NPs were prepared by the direct hydrolysis method,53 and used as the ETL in the inverted structure device (Figure S-3, S-4). Furthermore, none of any conjugated or non-conjugated polyelectrolyte was used to modify the work function of cathode or ZnO,9, 11, 14 and modified PEDOT:PSS58 was used as the HTL, thereby simplifying the device fabrication process. 3.3. Device Performance The optimized device performance parameters with both conventional and inverted structures under the illumination of AM 1.5G, 100 mW cm-2 are summarized in Table 1. Figure 3a shows the typical J-V curves of the above optimized devices. As shown in Table 1, the conventional device exhibited a moderate PCE of 8.06%, with Voc = 0.92 V, Jsc = 13.07 mA cm-2 and FF = 0.67. Note that the device with inverted structure exhibited a sharply enhanced PCE to 8.84% with an improved Jsc of 14.28 mA cm-2 and FF of 0.68. To the best of our knowledge, this PCE of 8.84% is the highest efficiency reported for inverted small molecule-based solar cells,28, 50, 52 and is also among the highest efficiencies reported of polymer-based solar cells.10, 15, 17, 59 The Voc and FF are identical in both devices, while the Jsc for the inverted device shows about 9% increasement compared with the conventional device, which is consistent with the optical simulation results discussed above.

Table 1. Device Performance Parameters for BHJ Solar Cells based on DRCN7T:PC71BM (w/w, 1:0.5) with Conventional and Inverted Structures. Voc

Jsc

PCEave

PCEmax

Rs

Rsh

(%)

(%)

Ω cm2

Ω cm2

7.75±0.31

8.06

6.76

1094

FF

Conventional

(V)

(mAcm-2)

0.91±0.01

13.07±0.10

0.65±0.02



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 30

(120 nm) Inverted 0.90±0.01

14.09±0.19

0.68±0.01

8.60±0.24

8.84

5.10

1113

(120 nm)

Figure 3. J-V curves (a) and EQE spectra (b) of the conventional device with structure of ITO/PEDOT:PSS/DRCN7T:PC71BM/ZnO NPs/Al and the inverted device with structure of ITO/ZnO NPs/DRCN7T:PC71BM/modified PEDOT:PSS/Ag under simulated 100 mW cm-2 (AM 1.5G) illumination. As shown in Table 1, the series resistance (Rs) for the conventional and inverted device is 6.76 and 5.10 Ω cm2, respectively. Meanwhile, the conventional and inverted devices exhibit similar shunt resistances (Rsh) of 1094. and 1113 Ω cm2. These data are consistent with the comparable FF for the conventional and inverted devices. The external quantum efficiency (EQE) spectra for the optimized devices based on


Page 11 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DRCN7T with conventional and inverted structures are shown in Figure 3b. The inverted structure device exhibits higher photo-electron conversion efficiency from 400 to 700 nm than that of the conventional device, with the highest EQE value reaching 68% at 455 nm while the conventional device showed a peak EQE of 62% at 470 nm. These results are consistent with the optical simulated real absorptions in the active layer of the corresponding devices (as shown in Figure S-5), and prove that the inverted structure could indeed improve the Jsc and therefore the PCE. 3.4. Mechanism Studies The increased Jsc in the inverted structure OPV devices could originate from the reduced non-geminate recombination or the increased absorption of photons, or a combination of both.15 In order to clarify the origin of the enhanced Jsc, we will investigate the recombination mechanisms for both the conventional and inverted device based on light-intensity-dependent current-voltage experiments,27, photocurrent

density-effective

voltage

analyses,60,

62

transient

60, 61

absorption

measurements63, 64 and electrical simulations65, 66 in the following section. 3.4.1. Light-intensity-dependent Current-Voltage Experiments Analysis of the current density-voltage characteristics at various light intensities could provide important information about the recombination mechanisms in organic solar cells. We measured the J-V curves for both conventional and inverted device under different light intensities from 5 to 100 mW cm-2 (as shown in Figure S-6). The normalized photocurrent density for the conventional and inverted devices at different light intensities is shown in Figure S-7, it is found that the charge collection efficiencies remain constant over a wide range of the applied voltages (from short circuit conditions to the maximum power output point) for both devices. This indicates that both the conventional and inverted devices exhibit similar recombination losses under optimized conditions, which are dominated by almost the same first-order recombination from the short-circuit condition to the open-circuit condition.27 The light intensity dependence of the Jsc could provide valuable insight into the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

charge recombination kinetics at short circuit condition. At the short circuit condition, if there is no non-geminate recombination, the Jsc should be correlated to the light intensity (P) by Equation 1 with α = 1,

J sc ∝ Pα (1) The light intensity dependence of the Jsc for the conventional and inverted devices is shown in Figure 4a. The fitted power law yield α for the conventional and inverted devices are 0.967±0.006 and 0.959±0.007, respectively, which indicates a negligible non-geminate recombination for both conventional and inverted devices. If the mean effective electron or hole drift length is smaller than the film thickness or the charge carrier mobilities are not balanced in the devices, the built-up of space charges will arise and non-geminate recombination will become considerable. When the build-up of space charges reaches a fundamental limit, the photocurrent will have a three-fourth power dependence on light intensity (α= 3/4).60 On contrary, when there is no built-up of space charges, suggesting that all the separated charges are collected under perfect conditions, the power law dependence will give a factor of 1.61 The experimental power law dependence of photocurrent vs. incident light intensity is shown in Figure 4b and 4c, at a high effective voltage (Veff = 2 V), the power exponents (α) for conventional and inverted devices are 0.968±0.007 and 0.962±0.007, respectively, very close to the perfect conditions. At the short circuit condition, the power exponents for the conventional and inverted device are 0.967±0.006 and 0.963±0.007. Meanwhile, at the low effective voltage (ca. 0.26 V) corresponding to an external bias of 0.73 V where the maximum power output is observed, the power exponents for conventional and inverted device still exhibit similar and high values of 0.962±0.006 and 0.961±0.006, respectively. All these suggest little built-up of space charge and similar non-geminate recombination for both conventional and inverted structure devices.


Page 12 of 30

Page 13 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Measured Jsc of DRCN7T:PC71BM solar cells with conventional and inverted structure devices plotted against light intensity on a logarithmic scale. The Jph,sat, Jph,sc, Jph,m plotted against with the light intensity for the conventional device structure (b) and inverted device structure (c). 3.4.2. Photocurrent Density-Effective Voltage Analyses In order to further investigate the causes of the improved Jsc in the inverted device over the conventional device, the photocurrent is plotted as a function of the effective applied voltage (Veff) in a wide reverse bias range under AM 1.5G illumination.60, 62 As shown in Figure 5a, for both conventional and inverted devices, Jph has a nearly linear dependence on the voltage at the low values of Veff, which reaches saturation when the effective voltage Veff arrives at ~ 2 V, suggesting almost all the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

photogenerated excitons are dissociated into free carriers and all carriers are collected at the electrodes without non-geminate recombination. With the assumption that all the bound exciton pairs are dissociated in this case, the maximum generation rate of bound exciton pairs per unit volume (Gmax) and the charge collection efficiency (Pc) for the conventional and inverted devices could be determined. Gmax was calculated from Equation 2, Jph,sat = qGmaxL

(2)

where q is the electronic charge and L is the thickness of the active layer (ca. 120 nm for both cases). The values of Gmax for the inverted device and conventional device are 9.21 × 1027 m−3 s−1 (Jph,sat = 14.76 mA cm−2) and 8.62 × 1027 m−3 s−1 (Jph,sat = 13.81 mA cm−2), respectively. Since Gmax is related to the maximum absorption of incident photons, the increased Gmax for the inverted device suggests that better absorption exists, which is also in agreement with the optical simulated Jsc,simu and experimental Jsc results as discussed above.


Page 14 of 30

Page 15 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Net photocurrent density (a) and charge collection efficiency (b) versus effective voltage characteristics of the DRCN7T:PC71BM based conventional and inverted devices under constant incident light intensity (AM 1.5G, 100 mW cm-2). As shown in Figure 5b, the exciton dissociation efficiency could also be obtained at the short circuit condition from Equation 3, Pc=Jph,sc/Jph,sat .

(3)

At short circuit conditions, the Jph is 95.76% and 96.54% of the saturation current density (Jph,sat) (Jph at Veff = 2 V) for the conventional and inverted devices, respectively, which indicates both devices exhibit very high exciton dissociation efficiencies. At the maximal power output condition, Jph/Jph,sat for the conventional and inverted device is 81.05% and 83.54%, respectively, further proves that very high charge collection efficiency and less non-geminate recombination loss exists in both device structures. 3.4.3. Transient Absorption Measurements To elucidate the recombination mechanisms of the conventional and inverted devices, transient absorption (TA) was performed on both devices in a reflection geometry.63, 64 Figure 6a and 6b shows the TA spectra from 5 ns to 1 µs delay for the conventional and inverted structure devices, respectively. The negative part spanning from ~600 to ~730 nm is attributed to the ground state bleaching (GSB) of DRCN7T molecules which is due to the state-filling of the excited states. The long-lived positive shoulder or peak at around 800 nm and 1150 nm are almost identical to the peaks in the quasi-steady-state photo-induced absorption (PIA) spectra (as shown in Figure S-8). Since the quasi-steady-state PIA monitors long lifetime species and both the 800 nm and 1150 nm have similar lifetime of hundreds to microseconds, we ascribe them to the photo-induced absorption of hole polarons in DRCN7T. Figure 6c and 6d shows the comparison of GSB and polaron dynamics between conventional and inverted devices monitored at 680 and 1150 nm, respectively. The polarons in both devices have almost the same decay rate indicating that the charge recombination in the conventional and inverted structure devices is invariant. Early time dynamics (fs-ns)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

also shows there is negligible change in transient dynamics (as shown in Figure S-9). These results further confirmed that the excitation dynamics of DRCN7T:PC71BM was not altered when the device structure was changed. Hence, the enhancement of performance could not be attributed to less recombination loss in inverted device.

Figure 6. TA spectra (ns-µs) for the conventional (a) and inverted devices (b), the dynamics at 680 nm (c) and 1150 nm (d) in both devices are overlaid for comparison. 3.4.4. Electrical Simulations The electrical models developed by Nelson el al65, 66 were also simulated on both device structures to further clarify the origin of the enhanced Jsc. It has been demonstrated by introducing an exponential density of tail trapping states below the band edge of polymer and fullerene, the device performance can be successfully modelled. Charge recombination is considered to be Shockley-Read-Hall (SRH) trap-assisted type recombination that free charges in transporting level annihilate with trapped charges in tail states.66, 67 The consideration is plausible as we observed a monomolecular recombination as shown in Figure S-7 over the simulated voltage range for both conventional and inverted devices. Considering the density of state (DOS) width is much narrower in small molecules than in polymers and the high carrier mobility in small molecules,68 only fullerene tail trap states are included in the modelling. Non-geminate recombination happens when free holes annihilate with


Page 16 of 30

Page 17 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


trapped electrons. The absorbed photon flux distribution follows Figure S-2 and multiplies by a reduction factor (due to an overestimated IQE in the electrical field simulation) so that the total absorbed photon is coincident with the maximum current output in experiment. A global fitting was carried out with different incident light intensities for both conventional and inverted devices. The fitting results are shown in Figure S-10, and Table S1 listed the main parameters extracted from the fitting. It is found that with only one set of parameters, both the conventional and inverted devices could be well-fitted based on the simulated absorbed photon flux. This indicates that the performance improvements could only come from a better electrical field distribution profile, where more photons are absorbed in the inverted devices while other factors are hardly affected. 3.5. Device Stability A preliminary investigation of the long-term stability of the inverted and conventional devices was also carried out. Both devices were encapsulated and stored in the ambient atmosphere (humidity: 65%), their photovoltaic performance was measured periodically by transferring the devices back into the argon-filled glove box. As shown in Figure S-11, the inverted device showed a remarkable stability with almost no obvious degradation after three months (the PCE is kept at 93% after 103 days). However, the PCE for the conventional device with ZnO NPs as the ETL decreased to 84% after 40 days under the same test conditions, while the conventional device with PFN as the ETL showed a significant decrease (the PCE reduced to 45% after 23 days) under the same test conditions. Also, the light soaking test was performed at the same conditions, and similar results were obtained (as shown in Figure S-12). The PCE for the inverted device was almost kept as a constant after continue illumination for 70 min, while the conventional device employed ZnO NPs as the ETL exhibits about 5% decrease during the same test condition. Therefore future work should focus on how to enhance the stability of the conventional devices.69 Based on our results, the SM-OPVs could achieve the comparable stabilities with P-OPVs by using inverted device structures, which exhibit great potential for the future commercial applications.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4. CONCLUSION Using an inverted structure, a PCE of 8.84% has been achieved for solution processed SM-OPV using ZnO NPs as the ETL without any conjugated or non-conjugated polyelectrolytes. The performance improvement for the inverted devices is attributed to the enhanced Jsc, which mainly origins from the increased light absorption in the inverted device, rather than the reduced non-geminate recombination compared to the conventional devices fabricated under similar conditions. This is validated from the studies of

the

optical

simulations,

light-intensity-dependent current-voltage

experiments, photocurrent density-effective voltage analyses, transient absorption measurements and electrical simulations. These results would be helpful for device fabrication and optimization towards future possible commercialization of SM-OPV.

ASSOCIATED CONTENT Supporting Information Details of light-intensity-dependent current-voltage experiments, 2D-Grazing incidence X-ray diffraction, etc. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Author contributions †G. Long and B. Wu contributed equally to this work.

ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support from the MOST (Grants 2014CB643502 and 2012CB933401), NSFC (Grants 91433101, 51422304 and 51373078), NSF of Tianjin City (Grant 13RCGFGX01121), PCSIRT (IRT1257), the NTU start-up grant M4080514, SPMS collaborative Research Award M4080536,


Page 18 of 30

Page 19 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ministry of Education AcRF Tier 2 grant MOE2013-T2-1-081 and the Singapore National Research Foundation through the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Programme, and thank beamline BL14B1 (Shanghai Synchrotron Radiation Facility) for providing beam time.

RERERENCES (1) Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009-20029. (2) Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Synthesis of Conjugated Polymers for Organic Solar Cell Applications. Chem. Rev. 2009, 109, 5868-5923. (3) Chen, W.; Nikiforov, M. P.; Darling, S. B. Morphology Characterization in Organic and Hybrid Solar Cells. Energy Environ. Sci. 2012, 5, 8045-8074. (4) Darling, S. B.; You, F. The Case for Organic Photovoltaics. RSC Adv. 2013, 3, 17633-17648. (5) Li, G.; Chu, C.; Shrotriya, V.; Huang, J.; Yang, Y. Efficient Inverted Polymer Solar Cells. Appl. Phys. Lett. 2006, 88, 253503. (6) Yip, H. L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994-6011. (7) Søndergaard,, R.; Helgesen, M.; Jørgensen, M.; Krebs, F. C. Fabrication of Polymer Solar Cells Using Aqueous Processing for All Layers Including the Metal Back Electrode. Adv. Energy Mater. 2011, 1, 68-71. (8) Şahin, Y.; Alem, S.; de Bettignies, R.; Nunzi, J. M. Development of Air



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Stable Polymer Solar Cells Using an Inverted Gold on Top Anode Structure. Thin Solid Films 2005, 476, 340-343. (9) Yang, T.; Wang, M.; Duan, C.; Hu, X.; Huang, L.; Peng, J.; Huang, F.; Gong, X. Inverted Polymer Solar Cells With 8.4% Efficiency by Conjugated Polyelectrolyte. Energy Environ. Sci. 2012, 5, 8208-8214. (10)

Guo, X.; Zhou, N.; Lou, S. J.; Smith, J.; Tice, D. B.; Hennek, J. W.;

Ortiz, R. P.; Navarrete, J. T. L.; Li, S.; Strzalka, J.; Chen, L. X.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Polymer Solar Cells With Enhanced Fill Factors. Nat. Photonics 2013, 7, 825-833. (11) Chang, J. H.; Wang, H. F.; Lin, W. C.; Chiang, K. M.; Chen, K. C.; Huang, W. C.; Huang, Z. Y.; Meng, H. F.; Ho, R. M.; Lin, H. W. Efficient Inverted Quasi-Bilayer Organic Solar Cells Fabricated by Using Non-Halogenated Solvent Processes. J. Mater. Chem. A 2014, 2, 13398-13406. (12) Tan, Z.; Zhang, W.; Zhang, Z.; Qian, D.; Huang, Y.; Hou, J.; Li, Y. High-Performance Inverted Polymer Solar Cells With Solution-Processed Titanium Chelate As Electron-Collecting Layer on ITO Electrode. Adv. Mater. 2012, 24, 1476-1481. (13) Chen, S.; Small, C. E.; Amb, C. M.; Subbiah, J.; Lai, T. h.; Tsang, S. W.; Manders, J. R.; Reynolds, J. R.; So, F. Inverted Polymer Solar Cells With Reduced Interface Recombination. Adv. Energy Mater. 2012, 2, 1333-1337. (14) Cheng, Y. J.; Hsieh, C. H.; He, Y.; Hsu, C. S.; Li, Y. Combination of Indene-C60 Bis-Adduct and Cross-Linked Fullerene Interlayer Leading to Highly


Page 20 of 30

Page 21 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Efficient Inverted Polymer Solar Cells. J. Am. Chem. Soc. 2010, 132, 17381-17383. (15)

He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced

Power-Conversion Efficiency in Polymer Solar Cells Using an Inverted Device Structure. Nat. Photonics 2012, 6, 593-597. (16) Li, C. Z.; Chang, C. Y.; Zang, Y.; Ju, H. X.; Chueh, C. C.; Liang, P. W.; Cho, N.; Ginger, D. S.; Jen, A. K. Y. Suppressed Charge Recombination in Inverted Organic Photovoltaics Via Enhanced Charge Extraction by Using a Conductive Fullerene Electron Transport Layer. Adv. Mater. 2014, 26, 6262-6267. (17) Liao, S. H.; Jhuo, H. J.; Cheng, Y. S.; Chen, S. A. Fullerene Derivative-Doped Zinc Oxide Nanofilm As the Cathode of Inverted Polymer Solar Cells With Low-Bandgap Polymer (PTB7-Th) for High Performance. Adv. Mater. 2013, 25, 4766-4771. (18) Kong, J.; Hwang, I. W.; Lee, K. Top-Down Approach for Nanophase Reconstruction in Bulk Heterojunction Solar Cells. Adv. Mater. 2014, 26, 6275-6283. (19) Liu, Y.; Zhao, J,; Li, Z.; Mu, C.; Ma, W.; Hu, H.; Jiang, K.; Lin, H.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293-5300. (20) Walker, B.; Kim, C.; Nguyen, T. Q. Small Molecule Solution-Processed Bulk Heterojunction Solar Cells. Chem. Mater. 2011, 23, 470-482. (21) Mishra, A.; Bauerle, P. Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology. Angew. Chem. Int. Ed. 2012, 51, 2020-2067.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(22) Lin,

Page 22 of 30

Y.; Li, Y.; Zhan, X. Small Molecule Semiconductors for

High-Efficiency Organic Photovoltaics. Chem. Soc. Rev. 2012, 41, 4245-4272. (23) Chen, Y.; Wan, X.; Long, G. High Performance Photovoltaic Applications Using Solution-Processed Small Molecules. Acc. Chem. Res. 2013, 46, 2645-2655. (24) Coughlin, J. E.; Henson, Z. B.; Welch, G. C.; Bazan, G. C. Design and Synthesis of Molecular Donors for Solution-Processed High-Efficiency Organic Solar Cells. Acc. Chem. Res. 2013, 47, 257-270. (25) Liu, J.; Walker, B.; Tamayo, A.; Zhang, Y.; Nguyen, T. Q. Effects of Heteroatom

Substitutions on the Crystal Structure,

Film Formation, and

Optoelectronic Properties of Diketopyrrolopyrrole-Based Materials. Adv. Funct. Mater. 2012, 23, 47-56. (26) Nikiforov, M. P.; Lai, B.; Chen, W.; Chen, S.; Schaller, R. D.; Strzalka, J.; Maser, J.; Darling, S. B. Detection and Role of Trace Impurities in High-Performance Organic Solar Cells. Energy Environ. Sci. 2013, 6, 1513-1520. (27) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Leong, W. L.; Ke, L.; Bazan, G. C.; Heeger, A. J. Intensity Dependence of Current-Voltage Characteristics and Recombination in High-Efficiency Solution-Processed Small-Molecule Solar Cells. ACS Nano 2013, 7, 4569-4577. (28) Kyaw, A. K. K.; Wang, D. H.; Gupta, V.; Zhang, J.; Chand, S.; Bazan, G. C.; Heeger, A. J. Efficient Solution-Processed Small-Molecule Solar Cells With Inverted Structure. Adv. Mater. 2013, 25, 2397-2402. (29) Shin, J.; Kang, N. S.; Kim, K. H.; Lee, T. W.; Jin, J. I.; Kim, M.; Lee, K.; Ju,


Page 23 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


B. K.; Hong, J. M.; Choi, D. H. J-Aggregation Induced Low Bandgap Anthracene-Based Conjugated Molecule for Solution-Processed Solar Cells. Chem. Commun. 2012, 48, 8490-8492. (30) Rousseau, T.; Cravino, A.; Ripaud, E.; Leriche, P.; Rihn, S.; De Nicola, A.; Ziessel, R.; Roncali, J. A Tailored Hybrid BODIPY-Oligothiophene Donor for Molecular Bulk Heterojunction Solar Cells With Improved Performances. Chem. Commun. 2010, 46, 5082-5084. (31) Shang, H.; Fan, H.; Liu, Y.; Hu, W.; Li, Y.; Zhan, X. A Solution-Processable Star-Shaped Molecule for High-Performance Organic Solar Cells. Adv. Mater. 2011, 23, 1554-1557. (32) Lee, O. P.; Yiu, A. T.; Beaujuge, P. M.; Woo, C. H.; Holcombe, T. W.; Millstone, J. E.; Douglas, J. D.; Chen, M. S.; Fréchet, J. M. Efficient Small Molecule Bulk Heterojunction Solar Cells With High Fill Factors Via Pyrene-Directed Molecular Self-Assembly. Adv. Mater. 2011, 23, 5359-5363. (33) Bürckstümmer, H.; Tulyakova, E. V.; Deppisch, M.; Lenze, M. R.; Kronenberg, N. M.; Gsänger, M.; Stolte, M.; Meerholz, K.; Würthner F. Efficient Solution-Processed Bulk Heterojunction Solar Cells by Antiparallel Supramolecular Arrangement of Dipolar Donor-Acceptor Dyes. Angew. Chem. Int. Ed. 2011, 50, 11628-11632. (34) Fitzner, R.; Mena-Osteritz, E.; Mishra, A.; Schulz, G.; Reinold, E.; Weil, M.; Körner, C.; Ziehlke, H.; Elschner, C.; Leo, K.; Riede, M.; Pfeiffer, M.; Uhrich, C.; Bäuerle, P. Correlation of π-Conjugated Oligomer Structure With Film Morphology



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 30

and Organic Solar Cell Performance. J. Am. Chem. Soc. 2012, 134, 11064-11067. (35) Kan, B.; Zhang, Q.; Li, M.; Wan, X.; Ni, W.; Long, G.; Wang, Y.; Yang, X.; Feng,

H.;

Chen,

Y.

Solution-Processed

Organic

Solar

Cells

Based

on

Dialkylthiol-Substituted Benzodithiophene Unit With Efficiency near 10%. J. Am. Chem. Soc. 2014, 136, 15529-15532. (36) Zhou, J.; Zuo, Y.; Wan, X.; Long, G.; Zhang, Q.; Ni, W.; Liu, Y.; Li, Z.; He, G.; Li, C.; Kan, B.; Li, M.; Chen, Y. Solution-Processed and High-Performance Organic Solar Cells Using Small Molecules With a Benzodithiophene Unit. J. Am. Chem. Soc. 2013, 135, 8484-8487. (37) Long, G.; Wan, X.; Kan, B.; Hu, Z.; Yang, X.; Zhang, Y.; Zhang, M.; Wu, H.; Huang, F.; Su, S.; Cao, Y.; Chen, Y. Impact of the Electron-Transport Layer on the Performance

of

Solution-Processed

Small-Molecule

Organic

Solar

Cells.

ChemSusChem 2014, 7, 2358-2364. (38) Wei, G.; Wang, S.; Sun, K.; Thompson, M. E.; Forrest, S. R. Solvent-Annealed Crystalline Squaraine:PC70BM (1:6) Solar Cells. Adv. Energy Mater. 2011, 1, 184-187. (39) Liu, Y.; Yang, Y.; Chen, C. C.; Chen, Q.; Dou, L.; Hong, Z.; Li, G.; Yang, Y. Solution-Processed Small Molecules Using Different Electron Linkers for High-Performance Solar Cells. Adv. Mater. 2013, 25, 4657-4662. (40) Wang, H.; Liu, F.; Bu, L.; Gao, J.; Wang, C.; Wei, W.; Russell, T. P. The Role of Additive in Diketopyrrolopyrrole-Based Small Molecular Bulk Heterojunction Solar Cells. Adv. Mater. 2013, 25, 6519-6525.


Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Shen, S.; Jiang, P.; He, C.; Zhang, J.; Shen, P.; Zhang, Y.; Yi, Y.; Zhang, Z.; Li, Z.; Li, Y. Solution-Processable Organic Molecule Photovoltaic Materials With Bithienyl-Benzodithiophene Central Unit and Indenedione End Groups. Chem. Mater. 2013, 25, 2274-2281. (42) Huang, J.; Zhan, C.; Zhang, X.; Zhao, Y.; Lu, Z.; Jia, H.; Jiang, B.; Ye, J.; Zhang, S.; Tang, A.; Liu, Y.; Pei, Q.; Yao, J. Solution-Processed DPP-Based Small Molecule That Gives High Photovoltaic Efficiency With Judicious Device Optimization. ACS Appl. Mater. Interfaces 2013, 5, 2033-2039. (43) Loser, S.; Bruns, C. J.; Miyauchi, H.; Ortiz, R. o. P.; Facchetti, A.; Stupp, S. I.; Marks, T. J. A Naphthodithiophene-Diketopyrrolopyrrole Donor Molecule for Efficient Solution-Processed Solar Cells. J. Am. Chem. Soc. 2011, 133, 8142-8145. (44) Li, W.; Kelchtermans, M.; Wienk, M. M.; Janssen, R. A. J. Effect of Structure on the Solubility and Photovoltaic Properties of Bis-Diketopyrrolopyrrole Molecules. J. Mater. Chem. A 2013, 1, 15150-15157. (45) Min, J.; Luponosov, Y. N.; Zhang, Z. G.; Ponomarenko, S. A.; Ameri, T.; Li, Y.; Brabec, C. J. Interface Design to Improve the Performance and Stability of Solution-Processed Small-Molecule Conventional Solar Cells. Adv. Energy Mater. 2014, 4, 1400816-1400824. (46) Zhang, Q.; Kan, B.; Liu, F.; Long, G.; Wan, X.; Chen, X.; Zuo, Y.; Ni, W.; Zhang, H.; Li, M. H. Z.; Huang, F.; Cao, Y.; Liang, Z.; Zhang, M.; Thomas, P. R.; Chen, Y. Small-Molecule-Based Solar Cells With Efficiency Over 9%. Nat. Photonics 2015, 9, 35-41.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(47) Liu, Y.; Wan, X.; Wang, F.; Zhou, J.; Long, G.; Tian, J.; You, J.; Yang, Y.; Chen, Y. Spin-Coated Small Molecules for High Performance Solar Cells. Adv. Energy Mater. 2011, 1, 771-775. (48) Li, Z.; He, G. R.; Wan, X. J.; Liu, Y. S.; Zhou, J. Y.; Long, G. K.; Zuo, Y.; Zhang, M. T.; Chen, Y. S. Solution Processable Rhodanine-Based Small Molecule Organic Photovoltaic Cells With a Power Conversion Efficiency of 6.1%. Adv. Energy Mater. 2012, 2, 74-77. (49) Long, G.; Wan, X.; Kan, B.; Liu, Y.; He, G.; Li, Z.; Zhang, Y.; Zhang, Y.; Zhang, Q.; Zhang, M.; Chen, Y. Investigation of Quinquethiophene Derivatives With Different End Groups for High Open Circuit Voltage Solar Cells. Adv. Energy Mater. 2013, 3, 639-646. (50) Lin, H. Y.; Huang, W. C.; Chen, Y. C.; Chou, H. H.; Hsu, C. Y.; Lin, J. T.; Lin, H. W. BODIPY Dyes With β-Conjugation and Their Applications for High-Efficiency Inverted Small Molecule Solar Cells. Chem. Commun. 2012, 48, 8913-8915. (51) Yamamoto, T.; Hatano, J.; Nakagawa, T.; Yamaguchi, S.; Matsuo, Y. Small Molecule Solution-Processed Bulk Heterojunction Solar Cells With Inverted Structure Using Porphyrin Donor. Appl. Phys. Lett. 2013, 102, 013305. (52) Shin, W.; Yasuda, T.; Hidaka, Y.; Watanabe, G.; Arai, R.; Nasu, K.; Yamaguchi, T.; Murakami, W.; Makita, K.; Adachi, C. π-Extended Narrow-Bandgap Diketopyrrolopyrrole-Based Oligomers for Solution-Processed Inverted Organic Solar Cells. Adv. Energy Mater. 2014, 4, 1400879-1400888.


Page 26 of 30

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(53) Beek, W. J. E.; Wienk, M. M.; Kemerink, M.; Yang, X.; Janssen, R. A. J. Hybrid Zinc Oxide Conjugated Polymer Bulk Heterojunction Solar Cells. J. Phys. Chem. B 2005, 109, 9505-9516. (54) Burkhard, G. F.; Hoke, E. T.; McGehee, M. D. Accounting for Interference, Scattering, and Electrode Absorption to Make Accurate Internal Quantum Efficiency Measurements in Organic and Other Thin Solar Cells. Adv. Mater. 2010, 22, 3293-3297. (55) Pettersson, L. A. A.; Roman, L. S.; Inganäs, O. Modeling Photocurrent Action Spectra of Photovoltaic Devices Based on Organic Thin Films. J. Appl. Phys. 1999, 86, 487-496. (56) Peumans, P.; Yakimov, A.; Forrest, S. R. Small Molecular Weight Organic Thin-film Photodetectors and Solar Cells. J. Appl. Phys. 2013, 93, 3693-3723. (57) Chen, K. S.; Salinas, J. F.; Yip, H. L.; Huo, L.; Hou, J.; Jen, A. K. Y. Semi-Transparent Polymer Solar Cells With 6% PCE, 25% Average Visible Transmittance and a Color Rendering Index Close to 100 for Power Generating Window Applications. Energy Environ. Sci. 2012, 5, 9551-9557. (58) Shi, H.; Liu, C.; Jiang, Q.; Xu, J. Effective Approaches to Improve the Electrical Conductivity of PEDOT:PSS: A Review. Adv. Electron. Mater. 2015, 1, 1500017-1500032. (59) Lu, L.; Xu, T.; Chen, W.; Landry, E. S.; Yu, L. Ternary Blend Polymer Solar Cells With Enhanced Power Conversion Efficiency. Nat. Photonics 2014, 8, 716-722. (60) Blom, P. W. M.; Mihailetchi, V. G. D.; Koster, L. G. J.; Markov, D. G. E.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 30

Device Physics of Polymer:Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551-1566. (61) Lenes,

M.;

Morana,

M.;

Brabec,

C.

J.;

Blom,

P.

W.

M.

Recombination-Limited Photocurrents in Low Bandgap Polymer/Fullerene Solar Cells. Adv. Funct. Mater. 2009, 19, 1106-1111. (62) Proctor, C. M.; Kim, C.; Neher, D.; Nguyen, T. Q. Nongeminate Recombination and Charge Transport Limitations in Diketopyrrolopyrrole-Based Solution-Processed Small Molecule Solar Cells. Adv. Funct. Mater. 2013, 23, 3584-3594. (63) Marsh, R. A.; Hodgkiss, J. M.; Friend, R. H. Direct Measurement of Electric Field-Assisted Charge Separation in Polymer:Fullerene Photovoltaic Diodes. Adv. Mater. 2010, 22, 3672-3676. (64) Marsh, R. A.; Hodgkiss, J. M.; bert-Seifried, S.; Friend, R. H. Effect of Annealing on P3HT:PCBM Charge Transfer and Nanoscale Morphology Probed by Ultrafast Spectroscopy. Nano Lett. 2010, 10, 923-930. (65) Kirchartz, T.; Pieters, B. E.; Kirkpatrick, J.; Rau, U.; Nelson, J. Recombination via Tail States in Polythiophene: Fullerene Solar Cells. Phys. Rev. B 2011, 83, 115209. (66) MacKenzie, R. C. I.; Shuttle, C. G.; Chabinyc, M. L.; Nelson, J. Extracting Microscopic Device Parameters From Transient Photocurrent Measurements of P3HT:PCBM Solar Cells. Adv. Energy Mater. 2012, 2, 662-669. (67) Shockley W, R. WT. Statistics of the Recombinations of Holes and Electrons.


Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Phys. Rev. 1952, 87, 835-842. (68) Zhong, C.; Huang, F.; Cao, Y.; Moses, D.; Heeger, A. J. Role of Localized States on Carrier Transport in Bulk Heterojunction Materials Comprised of Organic Small Molecule Donors. Adv. Mater. 2014, 26, 2341-2345. (69) Li, C. Z.; Chueh, C. C.; Yip, H. L.; O’ Malley, K. M.; Chen, W. C.; Jen, A. K. Effective Interfacial Layer to Enhance Efficiency of Polymer Solar Cells Via Solution-Processed Fullerene-Surfactants. J. Mater. Chem. 2012, 22, 8574-8578.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Content


Page 30 of 30

Enhancement of Performance and Mechanism Studies of All-Solution

Recommend Documents